Generally, an infrared-type gas detector is provided to detect a concentration of a detected gas according to an absorption amount of infrared which has a particular wavelength and absorbed by the detected gas. For example, with reference to JP-7-286809A and JP-2001-228326A, the infrared-type gas detector is provided with a Fabry-Perot filter.
The Fabry-Perot filter has a fixed mirror and a moving mirror which faces the fixed mirror and is separated from the moving mirror with a predetermined gap. A predetermined voltage is applied between a fixed electrode arranged at the fixed mirror and a movable electrode arranged at the moving mirror, so that the moving mirror is displaced. Thus, the infrared having a desirable wavelength corresponding to the gap which varies in response to the displacement of the moving mirror can be selected to pass. Therefore, the infrared-type gas detector having the Fabry-Perot filter can detect the infrared having multiple wavelength fields by a single infrared receiving element.
Moreover, as disclosed in JP-11-142245A, an infrared-type gas detector is provided with a Fabry-Perot filter and an infrared receiving element which are packaged in an inner space of a can. The inner space is hermetically closed.
However, in the case where the Fabry-Perot filter and the infrared receiving element are can-packaged, the pressure of the inner space will increase due to the increase of the temperature of the inner space because of the heating of the can package due to the infrared or the heat generation of a driving element (for driving Fabry-Perot filter, for example) arranged in the inner space. That is, unevenness will occur in the gap of the Fabry-Perot filter due to the variation (that is, variation of resistance due to gas) in the pressure of the inner space, even when the applied voltage is same.